ADHD is a common mental disorder that begins in childhood and can continue through adolescence and adulthood. It makes it hard for a child to focus and pay attention. Some children may be hyperactive or have trouble being patient. For children with ADHD, levels of inattention, hyperactivity, and impulsive behaviors are greater than for other children in their age group. ADHD can make it hard for a child to do well in school or behave at home or in the community.